Airborne particle pollution, especially particle matter size less than 2.5 μm diameter range (named “PM2.5”), is a big concern for countries like China, where the speed of industrialization stretches the boundaries of regulatory requirements.
As a consequence of increasing consumer empowerment, the demand for information about the air quality of living spaces is increasing. Especially in China, excessive PM2.5 pollution has become a common problem in the last decade. This problem is also validated by continuous measurements in various Chinese cities. The data is publicly available and can be simultaneously monitored by mobile phone applications or through the web.
Availability of this data as well as continuous national and international media attention has created strong consumer awareness about the problem.
Official outdoor air quality standards define particle matter concentration as mass concentration per unit volume (e.g. μg/m3). The average PM2.5 pollution concentration in mainland China has been calculated based on satellite data, and it has been found that the most of the country exceeds the World Health Organization limits of 10 μg/m3, with some regions reaching and even exceeding PM2.5 concentrations of 100 μg/m3.
Low cost particle sensors are for example based on the measurement of light scattered at the particles which are entrained by an air flow through a detection volume. The air flow is for example induced by a fan or a heater. Optical particle sensors typically give a particle count as the sensor output, and this information is then converted to a mass concentration. However, this conversion is usually not accurate, as different types of aerosols have different densities. Thus, the low cost optical approach does not easily enable discrimination between different particle sizes.
More expensive optical systems for professional use are available that also provide information on the size distribution of the particles in the air. However, these systems are more complex and expensive.
There are alternative sensor designs which collect particles and directly measure the total collected particle mass over time, for example using a gravimetric approach. Mass sensitive sensors do not however discriminate between the sizes of particles, and therefore do not provide information on the distribution of particle sizes in the air. Size specific particle detection (e.g. PM2.5) in conventional gravimetric systems therefore requires additional size classification approaches, such as cyclones, filters, impactors, etc., which increase the system complexity and generate a maintenance burden.
The mass measurement may for example be performed using a quartz crystal microbalance, a tapered resonator, an impactor, or weighing filters and sieves.
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